1. Field of the Invention
The present invention relates to a resonant frequency-temperature characteristics compensable elemental device, in particular to a device utilizing resonance phenomena of a dielectric ceramic such as resonator systems and oscillator systems of which the resonant frequency temperature coefficient can be compensated by heat-treating the dielectric ceramic used therein.
2. Description of the Prior Art
Cavity resonators, ring resonators, dielectric resonators or the like are used in oscillating systems and filters for communication using high-frequencies such as microwave and milimeter wave. Of these resonators, the dielectric resonators are used extensively by virtue of the advantages that they have good temperature stability of resonant frequencies and are suitable for miniaturization of devices.
High frequency circuit elemental devices comprising a dielectric resonator mounted in a casing include, for example, resonator systems, oscillators for stabilizing high frequencies and filters. It is required for such devices to have good temperature stability of resonant frequencies (or oscillating frequencies) as a whole. For example, local oscillators, one of said oscillators, are assembled by mounting a dielectric resonator, FETs, strip lines, etc. in a casing. In the case of this oscillator, it is required that the influences on the temperature characteristics of the dielectric resonator by the other parts such as FETs and the casing are compensated so that the device may have a temperature coefficient of 0 or so as a whole.
Recently, dielectric ceramics are extensively used as the dielectric resonator. In that case, the temperature coefficient (.tau..sub.f) of resonant frequency of a dielectric ceramic is fixed based on the composition of the dielectric ceramic. Accordingly, in order to enable the assembled device to have a desired temperature characteristics as a whole, it has been so far necessary to produce a great number of dielectric ceramics having diversity of .tau..sub.f in advance, to choose a ceramic with a suitable .tau..sub.f for assembly so that the influences by the other parts may be compensated.
The above method of assembly is, however, disadvantageous in that a great number of dielectric ceramics having diversity of .tau..sub.f must be produced in advance by changing the composition of individual ceramics. This is extremely troublesome.
The U.S. Pat. No. 4,731,207 discloses a process comprising the step of heating a green compact composed of a calcined product having a composition represented by the formula: EQU xBaO.yMgO.zTa.sub.2 O.sub.5
wherein x, y and z satisfy 0.5.ltoreq.x.ltoreq.0.7, 0.15.ltoreq.y.ltoreq.0.25, 0.15.ltoreq.z.ltoreq.0.25, and x+y+z=1, at a rate of from 100.degree. to 1,600.degree. C./min. up to a temperature of from 1,500.degree. to 1,700.degree. C., and subsequently retaining the green compact at the temperature for not less than 30 minutes. The ceramic produced by this process cannot undergo order-disorder transformation in crystal structure unlike the dielectric ceramic used in the present invention described later. Hence it is impossible to allow the temperature coefficient of the resonant frequencies to be changed by heat-treatment.